I thought so at first, but when 300 voters sign in and only 50 ballots are left in the box, I have to wonder what was wrong with the other 250 ballots that made them disappear.

I agree that a small mismatch between sign-in and ballot counts is just garden-variety carelessness. I'm very focused on ensuring that the counts stay even all throughout the day, and it's not easy. Some voters unaccountably wander out before hitting the final button, or even stalk out in a snit. You have to keep an eagle eye on everyone. The polling station has to be orderly and visible, and your clerks have to have a clue.